Broad Range Of Magnetic Fields Research Articles

1H Spin-Lattice Relaxation Processes in Solutions of H2N-Fe3O4 Nanoparticles: Insights from NMR Relaxometry.

1H spin-lattice relaxation experiments have been performed for water and glycerol/water solutions of H2N-Fe3O4 superparamagnetic nanoparticles (NPs) of about 7 nm diameter. The experiments encompass a broad frequency range covering 3 orders of magnitude, from 10 kHz to 10 MHz (referring to 1H resonance frequency), and have been performed in the temperature range from 298 to 313 K, varying the concentration of the superparamagnetic species. This extensive dataset has been used for twofold purposes. The first one is to serve as a challenge for thorough tests of theoretical models describing nuclear relaxation in solutions of superparamagnetic NPs, depending on their magnetic properties and dynamics of the solvent molecules. The challenge is posed by the wish to reproduce the data in a broad range of magnetic fields (not only at high fields) and by the need to explain the differences in the relaxation scenarios for water and glycerol/water solutions by varying only the solvent parameters. The second purpose is to get insights into the magnetic properties (electronic relaxation properties) of the nanoparticles due to their high applicational potential.

Colossal Linear Magnetoelectricity in Polar Magnet Fe_{2}Mo_{3}O_{8}.

The linear magnetoelectric effect is an attractive phenomenon in condensed matters and provides indispensable technological functionalities. Here a colossal linear magnetoelectric effect with diagonal component α_{33} reaching up to ∼480 ps/m is reported in a polar magnet Fe_{2}Mo_{3}O_{8}. This effect can persist in a broad range of magnetic field (∼20 T) and is orders of magnitude larger than reported values in literature. Such an exceptional experimental observation can be well reproduced by a theoretical model affirmatively unveiling the vital contributions from the exchange striction, while the sign difference of magnetocrystalline anisotropy can also be reasonably figured out.

Disorder-Enriched Magnetic Excitations in a Heisenberg-Kitaev Quantum Magnet Na_{2}Co_{2}TeO_{6}.

Using optical magnetospectroscopy, we investigate the magnetic excitations of Na_{2}Co_{2}TeO_{6} in a broad magnetic field range (0 T≤B≤17.5 T) at low temperature. Our measurements reveal rich spectra of in-plane magnetic excitations with a surprisingly large number of modes, even in the high-field spin-polarized state. Theoretical calculations find that the Na-occupation disorder in Na_{2}Co_{2}TeO_{6} plays a crucial role in generating these modes. Our Letter demonstrates the necessity to consider disorder in the spin environment in the search for Kitaev quantum spin liquid states in practicable materials.

Linear magnetoelectric memory and training effect in the honeycomb antiferromagnet Co4Nb2O9

We report observation of ultrarobust linear magnetoelectric memory and significant training effect in a honeycomb antiferromagnet ${\mathrm{Co}}_{4}{\mathrm{Nb}}_{2}{\mathrm{O}}_{9}$, which is controllable by magnetic and electric fields. The memory states show distinct linear magnetoelectric coefficients over a broad magnetic field range. Antiferromagnetic domain evolution is believed to be responsible for the versatile memory behaviors promisingly accessible in multiferroics and other magnetoelectric materials such as topological insulators. The compensated antiferromagnetic phase essential to the magnetoelectric memory may allow to further integrate the unique merits of antiferromagnetic spintronics such as ultrahigh density and ultrafast switching.

Magnetic damping of ferromagnetic and exchange resonance modes in a ferrimagnetic insulator

Understanding the damping is an important fundamental problem with widespread implications in magnetic technology. Ferrimagnetic materials offer a rich platform to explore not only the damping of the ferromagnetic mode, but also the damping of the high-frequency exchange mode very promising for ultrafast devices. Here we use time-resolved magneto-optical Kerr effect to investigate the ferromagnetic and exchange resonance modes and their damping in the bismuth-doped gadolinium iron garnet over a broad range of magnetic fields (0--10 T) and temperatures (50--300 K) including the magnetization and angular compensation points. These two resonance modes are excited via the inverse Faraday effect and unambiguously identified by their distinct frequency dependence on temperature and magnetic field. The temperature-dependent measurements in the external magnetic field ${H}_{\mathrm{ext}}=2\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ revealed that the intrinsic damping of the ferromagnetic mode is always smaller than the one of the exchange modes and both have a maximum near the angular compensation point. These results are fully consistent with recent predictions of atomistic simulations and a theory based on two-sublattice Landau-Lifshitz-Bloch equation. We also demonstrate that the damping of these modes varies differently as a function of ${H}_{\mathrm{ext}}$. We explain the observed behaviors by considering the different features of the effective fields defining the precession frequencies of the ferromagnetic and exchange modes.

Exact Christoffel-Darboux expansions: A new, multidimensional, algebraic, eigenenergy bounding method

Although the Christoffel-Darboux representation (CDR) plays an important role within the theory of orthogonal polynomials, and many important bosonic and fermionic, multidimensional, Hermitian and Non-Hermitian, systems can be transformed into a moment equation representation (MER), the union of the two into an effective, algebraic, eigenenergy bounding method has been overlooked. This particular fusion of the two representations (CDR and MER), defines the Orthonormal Polynomial Projection Quantization—Bounding Method (OPPQ-BM), as developed here. We use it to analyze several one dimensional and two dimensional systems, including the quadratic Zeeman effect for strong-superstrong magnetic fields. For this problem, we match or surpass the excellent, but intricate, results of Kravchenko et al (1996 Phys. Rev. A 54 287) for a broad range of magnetic fields, without the need for any truncations or approximations. The methods developed here apply to any linear, partial differential equation eigen-parameter problem, hermitian or non-hermitian.

Dynamic N14 nuclear spin polarization in nitrogen-vacancy centers in diamond

We studied the dynamic nuclear spin polarization of nitrogen in negatively charged nitrogen-vacancy (NV) centers in diamond both experimentally and theoretically over a wide range of magnetic fields from 0 to 1100 G covering both the excited-state level anti-crossing and the ground-state level anti-crossing magnetic field regions. Special attention was paid to the less studied ground-state level anti-crossing region. The nuclear spin polarization was inferred from measurements of the optically detected magnetic resonance signal. These measurements show that a very large (up to $96 \pm 2\%$) nuclear spin polarization of nitrogen can be achieved over a very broad range of magnetic field starting from around 400 G up to magnetic field values substantially exceeding the ground-state level anti-crossing at 1024 G. We measured the influence of angular deviations of the magnetic field from the NV axis on the nuclear spin polarization efficiency and found that, in the vicinity of the ground-state level anti-crossing, the nuclear spin polarization is more sensitive to this angle than in the vicinity of the excited-state level anti-crossing. Indeed, an angle as small as a tenth of a degree of arc can destroy almost completely the spin polarization of a nitrogen nucleus. In addition, we investigated theoretically the influence of strain and optical excitation power on the nuclear spin polarization.

EPR spectroscopy of neutron irradiated nanocrystalline boron nitride (h-BN) particles

Paramagnetic centers in the nanocrystalline boron nitride (h-BN) particles were comparatively studied before and after neutron irradiation. Electron Paramagnetic Resonance (EPR) spectroscopic analyzes were performed in the broad range of magnetic field from 0.05 to 0.55 T (500–5500 Gauss). The range of 0.3270–0.3370 T was additionally sweeped, where more paramagnetic centers were observed. The nature of the new paramagnetic centers formed as a result of neutron transmutation in the BN nanoparticles were examined by EPR spectra. Formation mechanism of VB and VN vacancies has been studied in detail by neutron transmutations.

Sublattice magnetization driven anomalous Hall resistance of FeCoGd amorphous films

Measurements of magnetization [M(H, T)] and anomalous Hall resistance [Rxy(H, T)] are performed over a broad range of magnetic field (H) and temperatures (T) on sputter deposited 10 nm thick films of (FeCo)1−xGdx. The Gd content (x) in the films was changed by varying the Gd source power from 20 W to 50 W, in steps of 5 W. The saturation magnetization (Ms) of these films at 300 K shows a distinct minimum for the source power of 40 W. Measurements of M(H) for the 40 W sample at several temperatures establish full compensation of the antiferromagnetically coupled magnetic sublattices of Gd and FeCo at Tcomp = 270 K ± 10 K. The approach to compensation is characterized by the emergence of perpendicular magnetic anisotropy (PMA) and a diverging coercive field. The Rxy (H) of this sample, as well as of those prepared at 30 W and 50 W, scales with M(H) at T > Tcomp and T < Tcomp. However, this scaling fails in the vicinity of Tcomp where the Rxy undergoes a sign reversal. Our analysis of these data in the framework of the existing models for Rxy(H, T) in ferrimagnets suggests that the role of spin disorder and its topological contribution to Rxy may be necessary to account for the observed behavior. A precise identification of Tcomp is also important to stabilize technologically useful non-trivial spin textures and PMA in these systems.

Dynamics of Ionic Liquids in Confinement by Means of NMR Relaxometry-EMIM-FSI in a Silica Matrix as an Example.

1H and 19F spin–lattice relaxation studies for 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide in bulk and mesoporous MCM-41 silica matrix confinement were performed under varying temperatures in a broad range of magnetic fields, corresponding to 1H resonance frequency from 5Hz to 30MHz.A thorough analysis of the relaxation data revealed a three-dimensional translation diffusion of the ions in the bulk liquid and two-dimensional diffusion in the vicinity of the confining walls in the confinement. Parameters describing the translation dynamics were determined and compared. The rotational motion of both kinds of ions in the confinement was described by two correlation times that might be attributed to anisotropic reorientation of these species.

Magnetotransport and internodal tunnelling in Weyl semimetals

Internodal dynamics of quasiparticles in Weyl semimetals manifest themselves in hydrodynamic, transport and thermodynamic phenomena and are essential for potential valleytronic applications of these systems. In an external magnetic field, coherent quasiparticle tunnelling between the nodes modifies the quasiparticle dispersion and, in particular, opens gaps in the dispersion of quasiparticles at the zeroth Landau level. We study magnetotransport in a Weyl semimetal taking into account mechanisms of quasiparticle scattering both affected by such gaps and independent of them. We compute the longitudal resistivity of a disordered Weyl semimetal with two nodes in a strong magnetic field microscopically and demonstrate that in a broad range of magnetic fields it has a strong angular dependence $\rho(\eta)\propto C_1+C_2 \cos^2\eta$, where $\eta$ is the angle between the field and the separation between the nodes in momentum space. The first term is determined by the coherent internodal tunnelling and is important only at angles $\eta$ close to $\pi/2$. This contribution depends exponentially on the magnetic field, $\propto \exp\left(-B_0/B\right)$. The second term is weakly dependent on the magnetic field for realistic concentrations of the impurities in a broad interval of fields.

Theoretical and computational framework for the analysis of the relaxation properties of arbitrary spin systems. Application to high-resolution relaxometry

A wide variety of nuclear magnetic resonance experiments rely on the prediction and analysis of relaxation processes. Recently, innovative approaches have been introduced where the sample travels through a broad range of magnetic fields in the course of the experiment, such as dissolution dynamic nuclear polarization or high-resolution relaxometry. Understanding the relaxation properties of nuclear spin systems over orders of magnitude of magnetic fields is essential to rationalize the results of these experiments. For example, during a high-resolution relaxometry experiment, the absence of control of nuclear spin relaxation pathways during the sample transfers and relaxation delays leads to systematic deviations of polarization decays from an ideal mono-exponential decay with the pure longitudinal relaxation rate. These deviations have to be taken into account to describe quantitatively the dynamics of the system. Here, we present computational tools to (1) calculate analytical expressions of relaxation rates for a broad variety of spin systems and (2) use these analytical expressions to correct the deviations arising in high-resolution relaxometry experiments. These tools lead to a better understanding of nuclear spin relaxation, which is required to improve the sensitivity of many pulse sequences, and to better characterize motions in macromolecules.

Field-induced pseudo-skyrmion phase in the antiferromagnetic kagome lattice

We study the effects of an in-plane Dzyaloshinskii-Moriya interaction under an external magnetic field in the highly frustrated kagome antiferromagnet. We focus on the low-temperature phase diagram, which we obtain through extensive Monte-Carlo simulations. We show that, given the geometric frustration of the lattice, highly non trivial phases emerge. At low fields, lowering the temperature from a cooperative paramagnet phase, the kagome elementary plaquettes form non-coplanar arrangements with non-zero chirality, retaining a partial degeneracy. As the field increases, there is a transition from this "locally chiral phase" to an interpenetrated spiral phase with broken $\mathcal{Z}_{3}$ symmetry. Furthermore, we identify a quasi-skyrmion phase in a large portion of the magnetic phase diagram, which we characterize with a topological order parameter, the scalar chirality by triangular sublattice. This pseudo-skyrmion phase (pSkX) consists of a crystal arrangement of three interpenetrated non-Bravais lattices of skyrmion-like textures, but with a non-(fully)-polarized core. The edges of these pseudo-skyrmions remain polarized with the field, as the cores are progressively canted. Results show that this pseudo-skyrmion phase is stable up to the lowest simulated temperatures, and for a broad range of magnetic fields.

Steady Shear Rheology and Magnetic Properties of Flake-Shaped Iron Particle-Based MR Fluid: Before and After Tribology Study

In this work, rheological properties and magnetic properties of flake-shaped iron particle-based magnetorheological (MR) fluid after tribology study were investigated and compared with that of before tribology study. With this aim, the flow properties of the MR suspensions in a steady shear regime were investigated, using a rotational rheometer with a parallel-plate measuring cell. The magnetic properties were studied using vibrational magnetometer. The rheological study was carried out for a broad range of magnetic fields. The results show that MR effect decreases for the fluid which is exposed to tribology study. The off-state viscosity and magnetization of the system increases. The surface morphology investigated using scanning electron microscopy shows that ball surface was ware and flake-shape particle morphology remains unaltered. The increase in off-state viscosity is attributed to the wear particles from the ball surface, which are of typically submicron sized as well as some particles from flake-shaped iron surface. These particles behave as a lubricant between the flake-shaped particles and reduce the particle-particle friction. As a result, the orientation ordering increases when the field is higher than frictional force. This results in increase in magnetization at higher fields. The oxidation stability of the iron particle is also confirmed form thermo gravimetric analysis of the particle. The present investigation suggests the use of flake-shape particle can avoid in-use thickening of MR fluid.

Understanding the methyl-TROSY effect over a wide range of magnetic fields.

The use of relaxation interference in the methyl Transverse Relaxation-Optimized SpectroscopY (TROSY) experiment has opened new avenues for the study of large proteins and protein assemblies in nuclear magnetic resonance. So far, the theoretical description of the methyl-TROSY experiment has been limited to the slow-tumbling approximation, which is correct for large proteins on high-field spectrometers. In a recent paper, favorable relaxation interference was observed in the methyl groups of a small protein at a magnetic field as low as 0.33 T, well outside the slow-tumbling regime. Here, we present a model to describe relaxation interference in methyl groups over a broad range of magnetic fields, not limited to the slow-tumbling regime. We predict that the type of multiple-quantum transition that shows favorable relaxation properties change with the magnetic field. Under the condition of fast methyl-group rotation, methyl-TROSY experiments can be recorded over the entire range of magnetic fields from a fraction of 1 T up to 100 T.

ESR Analysis of Industrial Needle-Coke Samples

Industrial needle-coke samples are studied. Their paramagnetic characteristics are determined over a broad range of magnetic fields. For some samples, no pronounced signal from the microwave resonator is observed. This indicates that the conversion of the molecular structure to coke is complete in those samples. For some samples, the ESR spectra contain two sharply resolved signals from centers associated with aliphatic and aromatic fragments. That indicates incomplete structural transformation.

EPR Spectroscopic Studies of Neutron-Irradiated Nanocrystalline Silicon Carbide (3C-SiC)

Nanocrystalline silicon carbide (3C-SiC) has been irradiated by neutrons (2× 1013n ⋅ cm−2s−1) up to 20 hours. Paramagnetic centers and their nature have been investigated comparatively before and after neutron irradiation. The Electron Paramagnetic Resonance (EPR) measurements were performed over the broad range of magnetic field from 0.05 to 0.55 T (500 to 5500 Gauss) in order to detect different paramagnetic centers. Simultaneously, EPR spectroscopy performed over the range from 0.3270T to 0.3370T which is most paramagnetic centers appears including free radicals. Neutron irradiation effects on the concentration of $V_{Si}^{-}$ and $V_{C}^{+}$ vacancies has been investigated. The number of paramagnetic centers for different values of g-factor has been calculated appropriate to local cases existed around 3300G. After neutron irradiation creation mechanism of paramagnetic centers in the nanocrystalline silicon carbide has investigated.

Low-frequency spin dynamics in the XY quantum spin ice Yb2Pt2O7

The XY pyrochlore Yb$_2$Ti$_2$O$_7$, with pseudo spin 1/2 at the Yb$^{3+}$ site, has been celebrated as potential host for the quantum spin ice state. The substitution of non-magnetic Ti with Pt gives Yb$_2$Pt$_2$O$_7$, a system with remarkably similar magnetic properties. The large nuclear gyromagnetic ratio ($\gamma_{n}/2 \pi = 9.15$~MHz/T) of $^{195}$Pt makes Yb$_2$Pt$_2$O$_7$ an ideal material for NMR investigation of its unconventional magnetic properties. Based on the $^{195}$Pt nuclear spin-lattice relaxation rate $1/T_1$ and the magnetic specific heat $C_{p}$ measured in a broad range of magnetic field $B_{ext}$, we demonstrate that the field-induced magnon gap linearly decreases with $B_{ext}$ but additional low energy mode of spin excitations emerge below $\sim 0.5$~T.

High-field magnetoconductance in La-Sr manganites of FM and AFM ground states

Large-grain La1–xSrxMnO3 ceramic samples of compositions x = 0.45 and 0.55, representing the ferromagnetic (FM) and A-type antiferromagnetic (AFM) ground states, were produced via classical sintering at 1500 °C of cold-pressed sol-gel prepared single-phase nanoparticles. Using the same precursors, nanogranular forms of both manganite ceramics were prepared by fast spark plasma sintering at low temperature of 900 °C, which limits the growth of crystal grains. The magnetotransport of both the bulk and nanogranular forms was investigated in a broad range of magnetic fields up to 130 kOe and analyzed on the basis of detailed magnetic measurements. Both the large-grain and nanogranular systems with x = 0.45, possessing a pure FM state with similar Curie tempereature TC ≈ 345 K), show nearly the same conductivity enhancement in external fields when expressed relatively to the zero-field values. This positive magnetoconductance (MC) can be separated into two terms: (i) the hysteretic low-field MC that reflects the field-induced orientation of magnetic moments of individual grains, and (ii) the high-field MC that depends linearly on external field. In the case of large-grain ceramics with x = 0.55, a partially ordered FM state formed below TC = 264 K is replaced by pure A-type AFM ground state below 204 K. This A-type AFM state is characterized by positive magnetoconductance that is essentially of quadratic dependence on external field in the investigated range up to 130 kOe. On contrary, the nanogranular product with x = 0.55 exhibits a mixed FM/AFM state at low temperatures, and, as a consequence, its magnetotransport combines the features of FM and A-type AFM systems, in which the quadratic term is much enhanced and clearly dominates at high fields. For interpretation of observed behaviors, the theory of grain-boundary tunneling is revisited.

Field-Induced Instability of a Gapless Spin Liquid with a Spinon Fermi Surface.

The ground state of the quantum kagome antiferromagnet Zn-brochantite, ZnCu_{3}(OH)_{6}SO_{4}, which is one of only a few known spin-liquid (SL) realizations in two or three dimensions, has been described as a gapless SL with a spinon Fermi surface. Employing nuclear magnetic resonance in a broad magnetic-field range down to millikelvin temperatures, we show that in applied magnetic fields this enigmatic state is intrinsically unstable against a SL with a full or a partial gap. A similar instability of the gapless Fermi-surface SL was previously encountered in an organic triangular-lattice antiferromagnet, suggesting a common destabilization mechanism that most likely arises from spinon pairing. A salient property of this instability is that an infinitesimal field suffices to induce it, as predicted theoretically for some other types of gapless SLs.